The trillion ton Larsen C iceberg was big, and may indirectly affect sea levels by uncorking land ice; but the Petermann Glacier in Greenland is more worrisome, and it just cracked.
“…even though this week’s new iceberg at Petermann is just 1/500th the size of the massive one that broke off the Larsen C ice shelf in Antarctica earlier this month, it could eventually have a much bigger effect on global sea levels. Scientists believe that if Petermann collapses completely, it could raise the seas by about a foot.” – Grist
Sea ice doesn’t directly change sea levels, though as it melts it reflects less light and allows the Sun to warm the water. Land ice is more troubling because it does add to sea levels. The effect is so strong that one glacier can have such a large effect. The Petermann Glacier may be the biggest on Greenland, but it isn’t the only large glacier and Greenland isn’t the only source of glaciers. Previous conservative estimates may have been overly-conservative.
“meltwater from Greenland is now the leading cause of global sea-level rise, increasing more than five-fold since 1993” – Grist